Exchangeable sodium pool size and sodium turnover in freshwater- and saltwater-acclimated ducks and gulls

Abstract

The exchangeable sodium pool size and its turnover rate were determined in ducks, Anas platyrhynchos, and gulls, Larus glaucescens, acclimated to freshwater and subsequently to 2/3 seawater. Saline acclimation led to a significant reduction in body mass in ducks but not gulls. Exchangeable body sodium per unit wet body mass was significantly greater for gulls (43 mmol∙kg⁻¹) than for ducks (30 mmol∙kg⁻¹) drinking freshwater. It was not significantly different between saltwater-acclimated ducks (48 mmol∙kg⁻¹) and gulls (44 mmol∙kg⁻¹). Saline drinking increased the sodium pool of ducks but not gulls. Similarly, the sodium flux per unit body mass was significantly higher for freshwater gulls (4.2 mmol∙kg⁻¹∙day⁻¹) than for ducks (2.1 mmol∙kg⁻¹∙day⁻¹). It was not significantly different between saltwater gulls (19 mmol∙kg⁻¹∙day⁻¹) and ducks (21 mmol∙kg⁻¹∙days⁻¹). Saline acclimation increased sodium flux by a factor of 4.5 for gulls and 10.4 for ducks. Freshwater-acclimated gulls had significantly higher levels of plasma sodium, chloride, and osmolality than did ducks. Saline acclimation increased the plasma osmolality of both species and the sodium and chloride concentrations of the plasma of ducks, and it decreased the hematocrit of ducks. Hematocrit and plasma osmotic and ionic concentrations were similar for saline-acclimated gulls and ducks.